1. Field of the Invention
The present invention relates to a charged particle apparatus, and more particularly to an immersion electromagnetic compound objective lens for a charged particle apparatus.
2. Description of Related Art
In semiconductor manufacture, pattern defects occur on the mask or wafer during the fabrication process, which reduce the yield to a great degree. Defect inspection, and defect review are widely used for yield management. High spatial resolution, high throughput and low radiation damage on specimen are the major determinants when judging their performance. For the defect inspection and review systems with high spatial resolution, Low-voltage Scanning Electron Microscopy (LVSEM) with Field Emission Source (FES) has been the core technology because of its high spatial resolution and low radiation damage on specimen.
Different from a conventional Scanning Electron Microscope (SEM), in a LVSEM, the examined specimen surface is scanned by a focused low-energy (<5 keV) electron beam or typically called as probe. The low-energy electron irradiation limits the probe/specimen interaction to a very small volume beneath the specimen surface. This feature reduces the radiation damage on the specimen such as pattern shrinkage on wafer resist layer and exit area of excited secondary emission electrons (SE) at the same time. The SE exit area is larger than the incident probe size because some SEs are excited by the backscattered primary electrons at some distance from the incident site. As a result, the ultimate spatial resolution of LVSEM is almost totally defined by the probe spot size.
In a SEM, the probe spot diameter D on specimen surface is determined by the diameter Di of source geometric image, spherical aberration disc diameter Ds, chromatic aberration disc diameter Dc, diffraction disc diameter Dd, and Coulomb effect disc diameter De. Their relationships can be simplified to addition in quadrature:D=√{square root over (Di2+Ds2+Dc2+Dd2+De2)}  (1.1)Each disc diameter is defined as:
                              D          i                =                  M          ·                      D            0                                              (        1.2        )                                          D          S                =                              1            4                    ⁢                                    C              SA                        ·                          α              3                                                          (        1.3        )                                          D          C                =                              1            2                    ·                      C            CA                    ·          α          ·                      dV                          V              0                                                          (        1.4        )                                          D          d                =                                            0.61              ·                              λ                α                                      ⁢            λ                    =                      12.26                                          V                0                                                                        (        1.5        )                                De        ∝                              1                          V              n                                ⁢                                          ⁢          0                <        n        <        1                            (        1.6        )            
Here CSA and CCA are spherical and chromatic aberration coefficients which mostly come from the magnetic objective lens. V0 and dV are electron energy and energy spread. α is beam half angle. All of these dependents are defined at the image plane which is located at the specimen surface. M is imaging system magnification and D0 is virtual source diameter of Field Emission source. λ is de Broglie wavelength. V corresponds to the electron energy from source to image plane.
Obviously, the chromatic aberration disc, diffraction disc and Coulomb effect become larger at low energy V. The only way to reduce the diffraction disc is using a bigger half angle, but unfortunately this will increase chromatic aberration disc and spherical aberration disc at the same time. Without Coulomb interaction, the smallest probe spot size is the best balance of these discs (1.2)˜(1.5) by appropriately choosing the half angle and the magnification. To further reduce the probe spot size or increase the probe current without increasing the probe spot size, the aberration coefficients must be reduced.
To reduce the Coulomb effect as much as possible, the widely used method is to initially accelerate the electrons to a high kinetic energy and subsequently decelerate the electrons to a desired low final landing energy just prior to impinging onto the specimen, please refer to R. F. W. Pease titled “Low Voltage Scanning Electron Microscopy”, Record of the IEEE 9th Annual Symposium on Electron, Ion and Laser Beam Technology, Berkeley, 9-11 may 1967, pp. 176-187, the entire disclosures of which are incorporated herein by reference. The deceleration of electron energy is realized by a retarding field in front of the specimen. Please refer to FIG. 1, a conventional objective lens 130 for LVSEM is shown, in which an excitation coil 132 will provide magnetic field through york 131, and a potential difference between electrode 133 and specimen 170 will provide a retarding field for decelerating a primary beam landing on a specimen 170.
The retarding field may either partially overlap or connect to the magnetic objective field, depending on the deceleration starts inside the magnetic objective, please refer to U.S. Pat. No. 4,785,176 entitled to Juergen Frosien et al. filed Mar. 27, 1987 and entitled “Electrostatic-magnetic Lens for Particle Beam Apparatus”, the entire disclosures of which are incorporated herein by reference, or from the rear portion of the magnetic objective, please refer to U.S. Pat. No. 6,194,729 entitled to Eugen Weimer filed Jul. 26, 1998 and entitled “Particle Beam Apparatus”, U.S. Pat. No. 6,855,938 entitled to Dirk Preikszas et al. filed Jul. 16, 2003 and entitled “Objective lens for an Electron Microscopy System and Electron Microscopy System”, U.S. Pat. No. 6,897,442 entitled to Igor Petrov filed Apr. 25, 2003 and entitled “Objective Lens Arrangement for Use in a Charged Particle Beam Column”, U.S. Pat. No. 7,067,807 entitled to Igor Petrov et al. filed Sep. 8, 2004 and entitled “Charged Particle Beam Column and Method of its Operation”, and U.S. Pat. No. 6,498,345 entitled to Eugen Weimer et al. filed Jun. 23, 1999 and entitled “Particle Beam Device”, the entire disclosures of which are incorporated herein by reference. Both are actually an electromagnetic compound objective. Because the retarding field partially acts like a negative electrostatic lens, it can partially compensate the aberrations of the pure magnetic objective. As a result, the electron deceleration actually provides an effective way to reduce the aberration coefficients as well as Coulomb effects.
For the specimen that can stand a little strong magnetic field such as the wafer or mask inspection, immersing the specimen in a strong magnetic field is much helpful to reduce the aberration coefficients. The magnetic objective with a magnetic circuit gap facing the specimen can provide a strong magnetic field immersion to the larger size specimen. However, such a design will require a larger coil excitation (product of coil turns and coil current), because only a part of the magnetic field located in front of the specimen surface takes part the particle beam focusing. A larger coil excitation incurs heat conduction and cooling issue, and solving this issue increases complexity and instability of the system. Additional publications include U.S. Pat. No. 4,785,176 (Frosien et al.); U.S. Pat. No. 6,897,442 (Petrov); U.S. Pat. No. 7,067,807 (Petrov et al.); U.S. Pat. No. 6,194,729 (Weimer); U.S. Pat. No. 6,855,938 (Preikszas et al.); U.S. Pat. No. 6,498,345 (Weimer et al.); and “Low Voltage Scanning Electron Microscopy” (R. F. W. Pease, Record of the IEEE 9th Annual Symposium on Electron, Ion and Laser Beam Technology, Berkeley, 9-11 May 1967, pp. 176-187), the entire disclosures of which are incorporated herein by reference.
Accordingly, to realize high spatial resolution and high throughput at the same time in defect inspection and review, an objective lens for LVSEM, which generates smaller aberrations and works at low coil excitation, is needed.